After the formation of barrier islands, sediment accumulation in the lagoon produces two major “surfaces” which form concurrently at the base and top of estuarine deposits. These surfaces range from less than one-half to more than 30 kilometers wide, and are separated by as much as 30 meters of estuarine sediment. The basal surface is erosional and is shaped by the shifting estuarine channels. Relief may be as much as several meters; the surface is undulatory and slopes gently seaward. Basal estuarine deposits are clayey sands and gravels. Gravels commonly are angular, and cut and fill structures and cross-stratification are abundant. The upper part of the estuarine deposits, the salt marsh sediments, are mainly clayey sands and silts, commonly cut by channels. The surface at the top of the estuarine sequence is depositional, the result of lagoonal filling which converts this area to a salt marsh. The upper limit of deposition is spring high tide. Relief may be several tens of meters as a result of channels.
Only the depositional surface could be considered a morphological “terrace”; however, during late stages of barrier island-estuarine development, when sediment supply may be insufficient to maintain the barriers, marine erosion may produce a wave cut and fill “terrace.” This type of “terrace” is rare in Quaternary deposits of southeastern United States because of modification and burial during the subsequent formation of the two major estuarine surfaces.